Proton Therapy
An objective of radiation therapy is to maximize and conform absorbed dose to a specified target volume that has been determined to contain diseased tissue, while, at the same time, minimizing dose to surrounding healthy tissue. The conformation of dose typically requires the use of multiple beams and control, dynamic or static, of the available geometric and dosimetric beam parameters. A unique advantage of charged particle beams, including protons or heavier ions, is the ability to control the penetration depth of the radiation dose, i.e., the Bragg peak location. This results in significant dosimetric advantages of even static proton beams compared to other forms of radiation delivery. The improved dose localization permits higher tumor doses with increased sparing of normal tissue doses. Thus, both an increase in tumor control and a reduction in radiation morbidity are expected. The relative energy deposition along the direction of the beam of single proton and photon beams is compared in FIG. 1. The upper curve 10 represents the dose delivered by a photon beam as a function of depth into the subject tissue. Upper boundary 12 of the filled region of the figure denotes a ‘pristine proton Bragg peak,’ the dose delivered by a mono-energetic beam of protons. The ‘spread out Bragg peak’ (SOBP) 14 arises due to a spread in photon energies. The blank region 16 between the curves 10 and 14 corresponds to excess dose delivered by a photon beam relative to a charged-particle beam.
Over 20 hospital-based facilities world-wide now treat patients with proton beams, and more are under development.
Conformality in proton dose distribution is improved through the use of Intensity Modulated Proton Radiotherapy (IMPT). This technique uses a dynamically intensity and position controlled narrow-focused “pencil” beam of protons to precisely control the dose at individual points in the target volume inside the patient. The beams have intensity distributions with full-width half-maximum (FWHM) dimensions typically between 5.0 to 10.0 mm. Some IMPT beam delivery uses a raster scanning technique, effectively painting layer by layer, modifying the range of the protons in between layers. Another form of IMPT is the spot scanning approach which deposits dose at all the required ranges at a given spot and then the beam is moved to the next spot. A typical magnetic scanning speed is about 20,000 mm/sec. This implies beam motion of 5 mm in 250 μsec.
In a typical scenario for IMPT, a treatment fraction is administered by delivering multiple beams. Each beam has a series of 10 or so different layers, each of which has a different proton energy or penetration depth. Each layer has a corresponding 2-D intensity or fluence profile. With a dose on the order of a centigray per layer, each layer will take on the order of 30 seconds to deliver and treatment fractions may last on the order of 5 minutes. The 30 second time frame is the period over which the spatial distribution of proton intensity must be controlled. During this period, the same nominal intensity pattern will be scanned across the patient multiple times, at least several times per second. The number and frequency of scans is such that each scan has a small enough dose that any errors in the scan can be corrected by subsequent scans.
Solid, inorganic scintillators have been used, in conjunction with radiation therapy, for absorbed dose measurement (as described, for example, by J. M. Schippers, S. N. Boon and P. van Luijk, “Applications in Radiation therapy of a scintillating screen viewed by a CCD camera,” Nucl Instr. and Meth. A 477, pp. 480-85 (2002), and S. N. Boon, thesis, “Dosimetry and quality control of scanning proton beams” (1998), and references therein, all of which are incorporated herein by reference. Similarly, solid scintillators have been applied in therapeutic beam profiling and computer tomography. In all these cases, the scintillator must either be positioned in the beam in place of the tissue to be irradiated, or disposed to receive the irradiating beam after it has traversed the tissue.
A gas scintillator has been described by G. Coutrakon et al. (“A beam intensity monitor for the Loma Linda cancer therapy proton accelerator,” Medical Phys., vol. 18(4), pp. 817-20 (1991)) for application to proton irradiation therapy, for monitoring overall beam intensity, and proving a real time feedback for beam intensity stabilization, however, the use of a gas scintillator imposes containment and other difficulties.
Requirements for Real-Time Tracing Detectors
Scanning proton or ion beams are used in conjunction with real-time imaging or tracking detector for monitoring the beam. Monitoring detectors currently employed in scanning proton therapy systems are briefly surveyed. As with doubly scattered proton therapy, there are several direct and indirect monitors of the beam fluence and energy, including at least one ionization chamber close to the exit port of the nozzle through which the protons are channeled to the irradiated tissue. Scanning systems also have monitors for the current in the deflection magnets in the nozzle.
It has been proposed to use indirect variables such as magnet current mentioned above. However, this current is not a direct measure of the exiting beam angle and position. The beam energy and the beam angle and position entering the deflection magnets also play a role. The nozzle lies at the end of a long beam train with many components, and it is typically many meters from the accelerator. Consequently, designers of scanned proton and ion beam facilities do not rely solely upon the magnet currents to monitor scanning beam position. A real-time imaging detector would thus be of great value in applications of proton beams.
Any detector is typically one of several redundant means of monitoring the system. It provides information for validating the spatially varying radiation dose that is delivered. It also provides a safety check to guard against instrument failure that could harm a patient undergoing treatment or damage the facility. Such detectors may advantageously provide real-time information that could be fed back to the control system for actively adjusting the beam to fine-tune the radiation dose.
Real-time detectors currently in use or planned for use have several deficiencies addressed by the present invention described below. An ideal detector would have a minimum of complexity, would be easily used in a hospital environment, and would introduce little material into the beam so as to minimize scattering. This detector would be fast enough and have a high enough resolution to detect the beam size and position and dose or intensity at the appropriate time scale. It should also have a reasonable lifetime and be easy to replace if necessary. Some semiconductor detectors are under development for this purpose. However, these are intrusive in that they significantly affect the proton beam and are of large complexity and thus cost. Moreover, they may not stand up to the intense radiation for very long. Ionization chambers and multi-wire detectors are currently in use for this purpose, as described by Badura et al, “Safety and Control System for the GSI Therapy Project,” (ICALEPCS ′97, 1997) which is incorporated herein by reference. Various aspects of their performance, however, such as their scattering, spatio-temporal resolution, and their longevity and replacement cost after radiation damage, are less than desirable.